Usually it works the other way around, that Europeans want to book through US agencies.
Sorry to say there is no way for us to say whether or not this is a "real deal" unless we actually saw it.
I do know that some river cruise lines get different prices for Euro river cruises than we see in the U.S. - but we don't know what your deal is for.
I would say the best thing to do is to try your best to establish whether or not this agency is legit. If it is then make sure you form of payment is not going to have a currency conversion charge that wipes out your savings differential.